illrequests.orderid is used to store ILL requets IDs that originate outside of Koha. For some ILL systems, this ID is very important and often used to identify requests. It would be good if this was displayed in the main table of ILL requests, and if the column was sortable.
Looks like this is fixed by bug 21063 which shows all columns from the illrequests table, and makes it possible to hide them.